There are several different types of packages being used in the microwave industry for discrete small signal components and devices. These packages range from 6 GHz to 20 GHz in limiting frequency and vary from high reliability ceramic packages to low cost plastic packages. There remains a desire for a small size package including hermetic sealing, surface mount capability and microstrip compatibility, operating frequency up to 22 GHz, and the ruggedness and reliability to meet JAN-S requirements for environmental testing, i.e., the level of testing required for space quality devices.
The problem has been that the requirements for these features tend to be mutually contradictory. To be hermetic and meet the JAN-S requirements, the package usually has to be fairly large in order to have good lead strength and to have room for a hermetic seal. Conversely, to operate up to 20 GHz the package must be made very small to prevent cavity oscillation. Also, the parasitic capacitance from lead to lead must be very low, on the order of 0.05 pF, but the dielectric constants of hermetic ceramic package materials are fairly high, so line coupling must be kept to a minimum. To operate well in the 20 GHz range, the leads must be close to 50 ohm transmission lines. But this imposes physical and electrical constraints on the sealing material and makes hermetic sealing more difficult to achieve. Such considerations have made design and manufacture of suitable packages difficult.
A variety of prior art packages have some, but not all, of such features. The one most similar to the present invention is described and illustrated in U.S. Pat. No. 4,722,137, which also includes a description of processes for making such a package. This patent also describes in its background portion several prior approaches to packaging high frequency electronic components.
Even with the improvements described in the '137 patent, some failures of hermetic sealing are still found upon testing to the JAN-S requirements. In other words, it is desirable to enhance the yield of acceptable packages as compared with the yield obtainable by manufacturing packages as described in the '137 patent.
A mode of failure sometimes observed in such packages is lack of a hermetic seal, particularly after thermal cycling. It is believed that microcracks form in the glass seal between the base and lid of such packages, permitting unacceptable leakage. It is, therefore, desirable to provide means for reducing incidents of such microcracks or other sources of decreased yield of packages similar to those described in the '137 patent, without detracting from the desirable electrical characteristics of these packages nor increasing manufacturing costs. It is desirable if the improved yield is obtainable with manufacture of the product on the same assembly processes presently used.